Baffled...

M

maxum man

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Joined
Sep 8, 2002
Messages
10
We have a nice and clean 1992 forester 190 islander 19' pontoon boat. It is powered by a Tohatsu M40C outboard. While I realize this is by no means meant to be a speed boat, with just my wife and myself on it it will do 15 to 16 mph (by gps). This past week we also had our grown children out with us. The boat would not get over 8 mph.
2 in front, me at the helm , and one in the back. If someone else went to the back the motor almost sounded like it was going to die, and speed and rpm's dropped to nothing. I have ordered a tach, But I am positive that the motor is turning no where near 5k RPM. Prop looks good, but i can not find any numbers on it. I was able to measure and seems to be 10.5 diameter. Trim pin was set at 2nd from full trim, moved it in one hole, did nothing.
I know this is alot, but any suggestions??

where can I find #'s on the prop ?
Would a Hydro-foil help ??

Thanks,
JohnnyB
 
MinUph

MinUph

Chief Petty Officer
Joined
Jun 5, 2011
Messages
466
Re: Baffled...

You're getting decent speeds with two adults. The prop must be good for that. With the added weight you probably need a different prop. If there is no numbers on it I can't tell you where else to look. Did you check the hub for numbers?
 
C

crb478

Lieutenant Junior Grade
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Dec 6, 2006
Messages
1,036
Re: Baffled...

Does it still run good with just the two of you? I am surprised that someone coming to the back stalled it, usually when the tips come up they can pick up speed. Pushing your motor down deeper must be more back pressure than it can easily handle. Get the tach and then do a compression test. Start with the basics compression over 100 and within 10% on all cylinders? Put the boat in the water and see if you are getting wot rpm's. Is your gas fresh? Lastly mark the prop to see if it is beginning to spin, but I doubt that is it since the motor was trying to stall.
 
G

Georgesalmon

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Apr 14, 2012
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1,793
Re: Baffled...

take off the prop and inspect the hub under the nut for prop size.
 
M

maxum man

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Sep 8, 2002
Messages
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Re: Baffled...

Yes, Gas is fresh. Have ethanol sta-bil in tank. Boat runs great. Replaced lower unit in 2012. When son walked to back, and motor really labored, he walked towards front and rpm's & speed instantly picked up. I do not believe I am getting max rpm's. Tach on order, will find out then.
I thank all for the help and suggustions
JohnnyB
 
M

Malibu Rudder

Petty Officer 2nd Class
Joined
Jul 10, 2012
Messages
115
Re: Baffled...

I do not believe the motor size has anything to do with the loss of speed when someone merely moves to the back of the boat. It your motor set too low? Are the toons submerging when someone is in the back of the boat? Toon diameter? Is there water in the toons(logs)? These are just some basic questions in order to help you lead to an answer. Good luck.
 
wgl333

wgl333

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Jan 11, 2011
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140
Re: Baffled...

Did you trim out the motor when this happened?
 
S

steelespike

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Apr 26, 2002
Messages
19,069
Re: Baffled...

I wonder if the motor may be too low.This could slow it slightly lightly loaded and might be so low that
water may be restricting air intake when weight is added at the stern.
If your not having any ventilation you may gain some speed just by raising the motor.
 
BrianMc

BrianMc

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Oct 2, 2010
Messages
177
Re: Baffled...

Have you figured out what prop your running yet? If your tubes aren't holding water I'm betting your prop is probably way over pitched. If you pitched it to run at the upper rpm range (5200-5800) running light you still only need around an 7-8 pitch prop. Here's a scenario for you.

If your running say an eleven pitch prop at 16 mph your looking at maybe 3400-3900 rpm depending on the slip ratio. Adding 2 more people to your toon might knock you down another 2-3 mph. Now we're down to 2800-3300 rpm. Well out of its power band. If moving one person to the rear drops the stern you probably don't have the minimum hp to climb up and level it out anymore. So now your drag is higher,with only half the available hp. Not to mention being at full throttle and low rpm your fuel/air ratio may be loading it up and choking it out.

Even with a smaller pitch prop you might look at ways to move some weight forward.
 
M

maxum man

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Joined
Sep 8, 2002
Messages
10
Re: Baffled...

Okay, Thanks for the answers, and ideas. We got out on the lake this week end and my tach finally came in (what a comedy of errrors.. Thanks USPS) . Calm day calm water, maximum rpm logged was 4190 rpm. I am going to believe that the prop is the problem, but I have to find out what size I have. If I take it off and take it to a prop shop, will they be able to tell me. then do I go with more or less pitch ??

Thanks in advance
 
L

lakegeorge

Senior Chief Petty Officer
Joined
Mar 19, 2002
Messages
660
Re: Baffled...

your prop will have the size stamped on it. It may be in the hub and you ill have to take it off t find it if it's not stamped on the outside. Lowering the pitch 1" will usually increase rpm's by 200.
 
rickdb1boat

rickdb1boat

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Jan 23, 2002
Messages
11,195
Re: Baffled...

I think steelespike is on the money. Especially when you say that "If someone else went to the back, the motor almost sounded like it was going to die". Can you post a picture of the boat in the water so we can see how low the engine sits? Just an FYI, that engine RPM range is 5200-5800. Best to prop toward the high side with a light load. But I still believe the answer is that the engine is mounted too low. Might also post a pic of the engine where it mounts to the transom.
 
J

joebob14

Petty Officer 2nd Class
Joined
Feb 27, 2010
Messages
170
Re: Baffled...

X2^^^^^^^^^^^^^^^^
could be the motor is mounted to low or as simple as just some water in the pontoons making the whole rear of the boat sit to low. I run a 40 hp Yamaha on a 20 ft party barge at 18-20 mph trimed all the way down all day with me my wife and kid. just like any other boat the anti cavitation plate should be about level with the bottom of the pontoons, any lower and your motor is causing to much drag, if that's right I would check the toons for water, you will hear it sloshing if you have someone pull the trailer forward and hit the brakes. Just a couple things I would check for free before throwing money at it.
 
Old Screwball

Old Screwball

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Jun 20, 2012
Messages
240
Re: Baffled...

WOW, I've got a 75hp Merc on my 18' Party Barge and the best I can get is 24.6mph (GPS) and this is my 3rd prop. Best speed was a 12 pitch cupped Mischigan Wheel prop. 24.6 was wife and I and 2 small kids. If I put my adult son and daughter in law I'm down to 19 mph. You guys must be on a faster lake ;)
 
MH Hawker

MH Hawker

Vice Admiral
Joined
Jul 13, 2011
Messages
5,516
Re: Baffled...

I also used to have a M40C on my 20 foot and it would hit 18 with 2 on board, It seams to me one of those perfect match up for the most part, and if its a Nissan prop the pitch is molded into the blade it should be a white prop.
 
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M

maxum man

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Joined
Sep 8, 2002
Messages
10
Re: Baffled...

The prop is white. We are going up to the lake again on Friday. I will look again for a number. I will also remove the plugs from the toons and see what if anything comes out. I have a side view of the boat, trying to figure how to post it, I will also take a photo of the back of the boat to show the mounting of the motor. This however, may not be the issue. only one set of holes, and same motor/hull combo since new.Vacation 2013 113.jpgView attachment 213157
 

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M

maxum man

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Joined
Sep 8, 2002
Messages
10
Re: Baffled...

On the top of each toon in the middle is a tube that is bent at a 90. These tubes do not seem to be plugged. Also there is a brass plug at the bottom rear of each tube. Is it safe/recommended to remove the plugs to check for water?
 
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